Abstract. Thymosin β4 (Tβ4) is expressed in developing tissue, where it stimulates cell differentiation and migration. Further, Tβ4 is expressed during molar development in mice, but the expression and function of Tβ4 in odontoblasts during mammalian tooth development have not yet been reported. Therefore, this study examined the expression and function of Tβ4 in differentiating odontoblasts during tooth development. As observed by immunohistochemistry, Tβ4 was expressed in the oral epithelium and inside cells of the tooth bud on embryonic day 15 (E15). Further, on E17, Tβ4 was expressed strongly in the dental lamina and oral epithelium, but only expressed in part of the cells in the outer and inner dental epithelium. Tβ4 was strongly expressed in the entire cytoplasm of odontoblasts on postnatal day 1 (PN1) and expressed intensively in the apical area of odontoblasts on PN4. Further, expression of Tβ4 was increased gradually in odontoblasts from PN1 to PN21. In an odontoblast cell line, MDPC-23, expression of Tβ4 mRNA and protein was increased strongly on day 4 and gradually decreased from day 14. The gene expression of dentin sialophosphoprotein (DSPP), bone sialoprotein (BSP), osteocalcin (OCN), osteonectin (ON), and collagen type I, related with mineralization, was significantly decreased in si-Tβ4/MDPC-23 during differentiation compared to that in MDPC-23 cells. Taken together, our results suggest that Tβ4 may be involved in oral epithelial cell proliferation at the initial stage of tooth development and regulates the expression and secretion of proteins during odontoblast differentiation.

This paper describes a combined experimental and numerical investigation aimed a t identifying the physical mechanisms and modeling errors that are the culprits for the lack of correlation between experimental measurementsand analytical predictions of the modal parameters of helicopter airframes. The identification of the major contributors to this lack of correlation is a n indispensable first step to actual modification and impmvement of modeling pmesses and tools. The study focuses on simple configurations: panels with lap and butt joints fastened with HI-LITE fasteners or rivets. Impact hammer tests were used to measure their natural frequencies, mode shapes, and damping ratios. The m u l t s wem compared with numerical predictions based oncurrent industry modeling practices. Next, shaker tesb wereused to investigate the nonlinear behavior of the fastened connections. The most significant discrepancy between measurements and predictions were due to modeling idealizations of the joints. Nonlinearities were also importanl though to a lesser extent.

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A companion paper has described an experimental investigation aimed a t identifying the physical mechanisms and modeling errors that are the culprits for the lack of cornlation between experimental measurements and analytical predictions of the modal parameters of helicopter airframes. Large discrepancies were ohsewed between measured and predicted natural frequencies that can he primarily assigned to the modeling errors associated with current industry practices. The present paper focuses on the development of simple, improved numerical models based on a lumped parameter approach which accounts for joint flexihilities by a set of concentrated linear and torsional springs whose stiffnesses are determined experimentally. In the proposed finite element model, the joint compliance is represented by beams of a circular cross-section whose diameter is evaluated from the measured joint stiffnesses. The frequencies predicted by this proposed numerical model were compared with the measured frequencies for panels with various types ofjoints and improved correlation was found when compared with current industry modeling practices.

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